Common stimulation techniques used to enhance production of hydrocarbon fluids from subterranean formations including hydraulic fracturing and acidizing. Typically a fracturing treatment fluid is injected into the formation at a pressure sufficiently high to cause the formation to fracture providing with a pathway for hydrocarbons to flow from the reservoir to the wellbore. Proppant materials are then carried into the fracture by the fracturing fluid, where they remain after the treatment is completed. Proppants have a dual role: (1) they hold the fracture open and (2) create a porous and permeable bed that results from granules packing after the fracturing pressure is released and the fracturing fluid is flowed back. The resulting permeable proppant bed enhances the ability of fluids to migrate from the formation to the wellbore through the fracture, making fracture conductivity the most important parameter in determining the degree of success of a hydraulic fracturing treatment.
With some formations, such as shale, the reservoir is drilled horizontally and then completed with multistage fracture treatments. These completion techniques consist of pumping a large number of stages into the horizontal well.
A key contributor to the success of stimulation is increased fracture surface area which allows more exposure to hydrocarbons in the formation, thereby leading to increased hydrocarbon productivity. This increase in the fracture surface area can be achieved by maximizing the created complex hydraulic fracturing network with far-field diversion. In addition, near-wellbore diversion is an important technique for ensuring uniform distribution of the treatment fluid across a full stage length.
During the life of the well one or more additional fracture treatments may be used to improve the well productivity from zones that were not initially produced. Such additional fracture treatments are known as refracturing treatments or refracs. To properly refracture a well, isolation of certain existing perforations is critical. The isolation is used to restrict or deny the perforations leading to the reservoir sections considered depleted from receiving subsequent fracturing fluids. Operations required to achieve this isolation can range from using a rig to setting physical barriers that redirect the fluid flow (mechanical isolation) to the use of specialized particulates placed in the flow stream to divert the treatment. These specialized particles integrated into the flow stream are commonly called “diverting agents,” “diverting materials,” or “diverters.” Used effectively, diverters can eliminate the need for a rig to provide temporary physical barriers, thus improving the economics of a workover operation.
Diverters are also used to optimize the production of hydrocarbons from low permeability zones within a fracture network by diverting the flow of well treatment fluids (such as fracturing fluids and acidizing fluids) from higher permeability zones to lower permeability zones. When pumped into open hole fractures or through perforated casing, such diverters bridge off and prevent additional fluid flow into higher permeability zones. As a result, well treatment fluids (such as fracturing fluids and acidizing fluids) flow into other areas of lesser conductivity. By increasing the surface area and increasing flow resistance within created channels within the fracture network, hydrocarbon productivity is enhanced.
Many materials have been used as diverting agents. These include water-insoluble and oil-soluble calcium salts of fatty acids, cellophane flakes, naphthalenes, crushed limestone, sodium tetraborate, oyster shells, gilsonite, paraformaldehyde, perlite, oil-soluble resins, rock salt, benzoic acid and most recently phthalic anhydride, polylactic acid (PLA) and polyimides. Such diverters, however, are less effective in the stimulation of horizontal wells which typically require multistage fracture treatments, especially in far field operations. Alternative diverters are therefore desired especially for use in the development of a fracture network far field from the wellbore as well as near the wellbore.